1. Field of the Invention
The present invention relates generally to solid-state imaging devices, and more particularly to a charge transfer device for use in an image sensing system including a video movie camera and an electronic still camera or the like.
2. Description of the Related Art
Recently, in place of the conventional vacuum pickup tubes, solid-state charge coupled device (CCD) image sensors have been increasingly used in video movie cameras or electronic still-image cameras as their imaging elements. Employment of such CCD image sensors provides several technical advantages such as down-sizing, higher operating reliability, and so forth.
The current developmental thrust of CCD image sensor technology is toward dramatic image-quality improvement. As the electronic cameras require higher image quality, the CCD image sensors become more critical in the photosensitivity. Unfortunately, presently available image sensors cannot catch up with such trend of high-sensitivity achievement. The main reason thereof is that the image sensors cannot eliminate successfully the generation of a leak current called "dark current" in the photosensitive charge packet storage cells. During an image pickup period, unless such dark current is eliminated successfully, the resultant effective signal charge packets decrease in amount to deteriorate the photosensitivity. If the dark current characteristic varies among the cells of an image sensor due to inherent deviation in the manufacturing process thereof, such variation will cause the signal charge packets to contain noise. The noise occurrence will lead to further deterioration of the sensitivity characteristic of the image sensor.
A solid-state amplifying imager using a bipolar transistor is disclosed, for example, in "A 310k Pixel Bipolar Imager," Nobuyoshi Tanaka et al., International Solid-State Circuits Conference (ISSCC) Digest of Technical Papers, 1989, at pp. 96-97 (FIG. 1). The bipolar image sensor has a semiconductive substrate of N type conductivity, which defines a plurality of cell-formation areas on its top surface. A semiconductor layer of P type conductivity is formed in each cell area on the substrate surface. The P type layer functions as a base region of the bipolar transistor, while the substrate serves as a collector of it. A heavily-doped N (N+) type layer is arranged within the P type base layer to serve as an emitter region of the transistor. When an incident light coming from a scene being photographed is introduced onto the bipolar cell structure, charge carriers (holes) are photoelectrically produced and stored (integrated) in the P type base layer. By applying a positive voltage to a gate electrode insulatively disposed above the P type base layer, the charge carriers are read from the N+ emitter layer to create a corresponding cell current amplified by the bipolar transistor.
With the bipolar imager, the P type base layer and the N type substrate are set in a reverse bias condition during both a charge-integration period and a read period. In particular, during the integration period, a certain reverse bias voltage is applied therebetween, which voltage is greater in potential than that used during the read period. Note here that the term "reverse bias" is defined as a specific biasing state with the positive voltage being applied to the N type substrate. If a positive voltage is applied to the substrate, the base and the substrate (collector) will be forward-biased, thus causing a current to flow in the bipolar transistor. This results in that the current is amplified by a known transistor action. When the positive voltage is at 0.6 volts, the transistor current is almost saturated. The forward-current saturation region of the voltage-to-current characteristic diagram has been conventionally employed as a signal charge packet reading region. In this case, the saturation current occurring in the cell structure acts as the leak current or dark current Id. From the above explanation, it may be understood that it is inherently very difficult to eliminate the dark current Id from the bipolar imager, although such has been long desired among those skilled in the image sensor art. If the dark current potentially varies among a number of cells, the elimination of noise will become more difficult, which is a serious bar to the achievement of the high-sensitivity imager.
It may be considered that the generation of leak current can be suppressed by causing the reverse bias voltage to be less than 0.6 volts, and that the leak current can be eliminated almost completely under a zero-bias condition. However, such a narrow-ranged idea is not employed by a skilled person in the image sensor art. The reason is simple: Any bipolar imager can no longer operate normally under such an abnormal biasing condition. If the "lesser" bias condition is employed, the N type substrate serving as the transistor collector region must be kept at a lower potential while charge carriers are being read from the P type base layer by applying a drive voltage to the gate electrode. As far as the collector potential remains lower, the carriers that should flow toward the N+ emitter layer run away uncontrollably toward the collector layer. The effective signal charge packets are thus reduced in amount accordingly. If a zero-bias condition is employed, no signal charge carriers can be obtained.
For the above reasons, it has been recommended that the bipolar imagers are driven by providing their photosensitive cells with a reverse bias voltage of 2 volts or more. As far as such drive system is used, the bipolar imagers are compelled to suffer from the payment of vicarious compensation for increase in the dark current and decrease in the photosensitivity.